In recent years, phosphosphingolipids such as sphingomyelins (SPMs) are gaining interest for pharmaceutical and therapeutic applications.
Sphingolipids and especially SPMs are unique in their chemical stability. Lacking ester bonds and polyunsaturated acyl chains they resist hydrolysis and oxidation during storage and formulation processing. Therefore, SPMs are excellent candidates for drug delivery formulations based on liposomes and other lipid assemblies. Having full control over the composition one can design SPMS which, when present in a lipid bilayer under physiological conditions (e.g. at body temperature) may be in a fluid (i.e., N-oleoyl sphingomyelin), or solid (i.e., N-stearoyl sphingomyelin) state, or design SPMs which enable the generation of thermo-sensitive liposomes.
Another unique feature of SPMs is their high affinity for cholesterol thereby serving as potential drug to induce reverse cholesterol transport in cardiovascular diseases.
Initially, SPMs were obtained by extraction of animal tissue and further purification. But in the last two decades several synthetic strategies have been suggested to prepare SPM and related compounds.
When synthesized correctly, a sphingomyelin is a single molecular species composed of only one sphingoid base of a D-erythro configuration and one acyl chain (e.g. D-erythro N-palmitoyl sphingomyelin). Such SPMs are mainly obtained from milk or egg yolk and are present therein at very low concentrations. As a result, the extract is typically contaminated with other lipids, such as 1-alkyl-sn-glycerophoshoethanol amine and 1 alkyl-sn-glycerol phosphocholine [Do, U. H. and Ramochardarn, S. (1980) J. Lipid Res. 21, 888-894].
In addition, the extract may contain lipid contaminants which are resistant to known purification procedures. For example, milk derived lipids include a mixture of SPMs such as neutral glycosphingolipids and gangliosides [Martin, M. J., et al. (2001) J. Dairy Sci. 84, 995-1000; Martin, M. J., et al. (2001) Lipids 36, 291-298], being resistant to alkaline hydrolysis and thus glycosphingolipids may contaminate milk-derived sphingomyelin. As appreciated by those versed in the art, glycosphingolipids, like peptides and proteins, may be immunogenic and thus their present in the extract is not preferable.
There are also reports that lipids derived of milk (including milk derived sphingomyelin) may be contaminated with bacterial products such as from Streptococcus agalacial [Bendle, P. and Vuyletelova, M. (1997) Vet. Med. 42, 71-80].
In addition, milk-derived SPM and egg-derived SPM are known to include mixtures of SPMs which vary in their acyl chains [see for example Avanti Polar Lipids Inc. Products Catalog Edition VI, p. 58]. Typically, milk-derived SPMs are enriched with C24:0>C18:1>C16:0>>C18:0 and contain many other acyl chains. The very high percentage of these long acyl chains and therefore large mismatch between the two hydrocarbon chains makes this SPM very different from the egg-derived SPMs. The level of chain mismatch is a very important parameter in determining the physicochemical properties of SPMs [rev. in Barenholz, Y. and Thompson, T. E. (1999) Chem. Phys. Lipids 102, 29-34; Barenholz, Y. and Thompson, T. E. (1980) Biochim. Biophys. Acta 604, 129-158; Barenholz et al. (1976) Biochemistry 15, 2441-2447]. A major difference in the ability of the two SPMs to suppress intestinal cholesterol absorption by decreasing thermodynamic activity of cholesterol monomers was recently observed [Eckhardt, E. R., et al. W (2002) Gastroenterology 122, 948-956]. In addition, the SPMs derived of natural sources have more than one sphingoid base. Although C18 D-erythro sphingosine is the main sphingoid base, other sphingoid bases accompany the main base in significant percentage. Especially the sphingoid base dihydrosphingosine (which is saturated and lacks the trans double bond between C4-C5), and smaller amounts of sphingosine and dihyrosphingosine bases other than C18 [Morrison, W. R. and Hay, J. D. (1970) Biochim. Biophys. Acta 202, 460-467]. Both egg yolk derived SPM and milk-derived SPM sphingoid base and acyl chain composition is affected by diet and therefore batch to batch variation in sphingoid and acyl chain composition may occur and should be carefully studied. Such changes may also be reflected in the physicochemical and biological properties of the different batches.